According to the European Water Framework Directive (WFD), chemical water quality is assessed by monitoring 45 priority substances. However, observed toxic effects can often not be attributed to these priority substances, and therefore there is an urgent need for an effect-based monitoring strategy that employs bioassays to identify environmental risk. Algal photosynthesis is a sensitive process that can be applied to identify the presence of hazardous herbicides in surface water. Therefore, the aim of this study was to employ an algal photosynthesis bioassay to assess surface water toxicity to algae and to identify the compounds causing the observed effects. To this purpose, Raphidocelis subcapitata was exposed to surface water samples and after 4.5 h photosynthetic efficiency was determined using PAM fluorometry. In this rapid high throughput bioassay, algal photosynthesis was affected by surface water from only one of 39 locations. Single compounds toxicity confirmation elucidated that the observed effect could be solely attributed to the herbicide linuron, which occurred at 110 times the EQS concentration and which is not included in the WFD priority substances list. In conclusion, applying the algal photosynthesis bioassay enables more efficient and effective assessment of toxicity to primary producers because it: (i) identifies the presence of herbicides that would be overlooked by routine chemical WFD monitoring, and (ii) avoids redundant chemical analyses by focusing only on (non-)target screening in samples with demonstrated effects.

根据欧洲水框架指令（WFD），通过监测45种优先物质来评估化学水的质量。然而，观察到的毒性作用通常不能归因于这些优先物质，因此迫切需要一种基于效果的监测策略，该策略采用生物测定法来识别环境风险。藻类光合作用是一个敏感的过程，可用于识别地表水中有害除草剂的存在。因此，本研究的目的是采用藻类光合作用生物测定法评估地表水对藻类的毒性并鉴定引起所观察到影响的化合物。为此， Raphidocelis subcapitata将其暴露于地表水样品中，并在4.5小时后使用PAM荧光测定法测定光合作用效率。在这种快速的高通量生物测定中，藻类的光合作用仅受到39个位置之一的地表水的影响。单一化合物的毒性证实阐明，观察到的效果可能完全归因于除草剂亚麻嘧啶，其发生在EQS浓度的110倍且未包括在WFD优先物质清单中。总之，应用藻类光合作用生物测定法可以更有效地评估对初级生产者的毒性，因为：（i）确定常规化学WFD监测会忽略除草剂的存在，并且（ii）通过集中精力避免多余的化学分析仅在已证明有效果的样品中进行（非）目标筛查。